It is required that he ________ the payment immediately.

The correct choice, "send," is appropriate in this context due to the structure of the subjunctive mood used in English. The phrase "It is required that he" sets up a condition that expresses necessity or desire, often followed by the base form of the verb. In this case, "send" functions as the base form, indicating that he is required to take action regarding the payment without any modifications to the verb.

Using "sent" or "sended" would be incorrect because they do not align with the subjunctive structure. "Sends" is a conjugated form for the third person singular, which does not fit in this nominal clause where the base form is needed to convey requirement or urging. Hence, "send" is the only form that accurately completes the sentence while following proper grammatical rules.